The desiderata for a commercial high voltage, high power DC power supply are the universal commercial requirements of relatively low cost, small volume and weight, high reliability, safety, as well as ease of repair and manufacture. In general, prior art high voltage, high power DC power supplies have not met all of these goals.
This is particularly true for power supplies having voltage ratings in excess of 50kV and power ratings between 15kW and 60kW. With the typical prior art high voltage, high power DC power supplies, volume increases as a geometric function of voltage and power.
The conventional, prior art power supplies which have attempted to achieve the aforementioned voltage and power requirements are generally classified as (a) voltage multipliers, (b) layer wound high voltage transformers having high step-up ratios which directly drive a rectifier, and (c) hybrid combinations of the voltage multipliers and layer wound high voltage transformers.
Voltage multipliers include a transformer having a step-up secondary transformer for driving several voltage rectifying and multiplying stages, each of which includes plural capacitors, and plural diodes. The transformer has a primary winding driven by an AC source, typically having a voltage on the order of several hundred volts. To achieve an output voltage of, for example, 150kV, a prohibitive number of stages is required. The DC voltage derived from such multipliers has high ripple content, as well as poor regulation. The series capacitors must have large values, therefore are costly and physically large. The large capacitors store a large amount of energy which tends to be harmful to the power supply, its load, and personnel. The stored energy is particularly harmful while a load, such as an X-ray tube, is arcing.
The connections of the prior art multipliers require all stages to be in proper operating condition to obtain an output voltage. To derive control signals for an inverter that drives the multiplier a high voltage divider must be employed The voltage across each multiplier stage differs, so that the voltage from one stage cannot be used as a sample for control of the inverter.
In a typical converter including a layer wound transformer, an AC voltage of several hundred volts is applied to a primary winding of a transformer. The transformer typically must have a turns ratio on the order of 500 to 1 to achieve a DC output voltage on the order of 150kV. The AC input to such a transformer cannot exceed a few kHz because the transformer has substantial parasitic reactances, in the form of a large series leakage inductance and large shunt winding capacitances. The series leakage inductance and shunt capacitances form a low pass filter that causes the frequency supplied to the transformer to be relatively low. Low frequency operation usually requires large filters to smooth DC voltages resulting from rectification of the transformer output. The filters employ cores having large cross-sectional areas. Hence, such transformers are usually large and heavy and are not well suited to be mounted on a rotary gantry carrying an X-ray tube having high voltage and high power requirements.
The windings of such a transformer require a large window area due to dielectric constraints for creep and puncture effects. (The creep effect is the tendency for a breakdown to occur along the surface of a conductor as a result of a voltage difference subsisting at different locations along the length of a surface, while the puncture effect is a breakdown that occurs transversely of two surfaces at differing potentials.) Dielectric constraints force the use of transformers having large magnetic cores, which are not usually available in ferrite materials. Parasitic reactances of the transformer can also adversely affect components of a power inverter used to derive the AC which is supplied to the transformer primary winding. The leakage inductance of such a transformer stores sufficient energy to threaten semiconductor switches of the inverter.
If the frequency applied to such a transformer is in excess of a few kHz, the capacitance in the secondary winding circuit of such a transformer, as reflected to the primary winding of the transformer, is virtually a short circuit for the inverter connected to drive the primary winding. This is because the secondary capacitance is reflected to the primary winding by a multiplication factor equal to the square of the 500 to 1 turns ratio, so that, for example, a 100 picofarad parasitic capacitance in the secondary winding is reflected to the primary winding as a 25 microfarad capacitance. If the AC source driving the primary winding has a voltage of 300 volts and a frequency of 20kHz, the reflected parasitic capacitance would draw 942 amperes, approximately six times the current drawn by a 40kW load. Multilayer, high turns ratio transformers also, in many instances, have very high ratios of AC to DC resistance, resulting in very high power (I.sup.2 R) secondary winding losses.
A structure which is a combination of the voltage multiplier structure and the layer wound transformer structure is known as a hybrid multiplier/transformer structure. Such a structure includes a central primary winding and multiple concentric secondary windings at different radii from the primary winding. Each secondary winding includes a first terminal connected to opposite polarity electrodes of a pair of series connected diodes and a second terminal connected to a common connection of first and second series connected capacitors. A DC voltage is developed across the remaining electrodes of the diodes and capacitors. Several of these units are stacked together to develop the desired high DC output voltage.
The hybrid multiplier/transformer structure is an optimal use of a layer wound high voltage transformer. The hybrid multiplier/transformer minimizes AC stress on the transformer secondary windings because layer to layer stress within the transformer is only the different DC voltages between the stacked units. In addition, the hybrid multiplier/transformer structure solves some of the previously described capacitive problems.
Because the hybrid multiplier/transformer includes a central primary winding and plural concentric secondary windings some of the high voltage secondary windings are somewhat remote from the primary windings. There is less magnetic coupling between the remote secondary windings and the primary winding than between the primary winding and secondary windings proximate the primary winding. In consequence, the voltage and power contributions of the outer windings are reduced. The hybrid multiplier/transformer arrangements are impractical for high power and high step-up ratios, such as are required to achieve DC voltages of 150kV at powers between 15 and 60kW.
It is, accordingly, an object of the present invention to provide a new and improved high voltage, high power DC power supply having relatively small size, weight and cost.
Another object of the invention is to provide a new and improved high voltage, high power DC power supply, which can be effectively energized by an AC source having a frequency in excess of about 100kHz.
An additional object of the invention is to provide a new and improved high voltage, high power DC power supply having a DC output with low ripple and which achieves its rated voltage and power relatively quickly, e.g., in less than 100 microseconds.
A further object of the invention is to provide a new and improved high voltage, high power DC power supply having relatively low stored energy, thereby leading to increased personnel and equipment safety.
An additional object of the invention is to provide a new and improved high voltage, high power DC power supply having a relatively wide bandwidth.
Still a further object of the invention is to provide a new and improved high voltage, high power DC power supply having predictable and controlled electric field stresses.
Still another object of the invention is to provide a new and improved high voltage, high power DC power supply having relatively small primary to secondary capacitance, so that in response to load arcing there is a reduced threat to components, particularly switches, of an inverter which energizes the supply with high frequency AC (in excess of about 100kHz).
An additional object of the invention is to provide a new and improved high voltage, high power DC power supply that is arc tolerant as a result of AC and DC high voltage grading along the length of the supply remaining the same during load arcs.
Still an additional object of the invention is to provide a new and improved high voltage, high power DC power supply having reduced electromagnetic interference.
A further object of the invention is to provide a new and improved high voltage, high power DC power supply that is relatively easy to manufacture and maintain, and wherein a failure of one section of the power supply does not result in the complete inoperability of the entire supply.
To achieve the power requirements for an X-ray tube mounted on a rotating gantry for CT scanning applications, i.e., a 150kV difference between the tube anode and cathode at powers between 15 to 60kW, the prior art has employed a floor mounted structure for deriving a pair of 75kV outputs. The outputs of the structure maintain the cathode at -75kV and the anode at +75kV. Connections from the structure to the tube anode and cathode electrodes are via high voltage slip ring assembly on the rotating gantry. High voltage slip ring assemblies are expensive, awkward, difficult to design, have questionable reliability and are intermittent due to arcs. The only prior art X-ray power supply of which I am aware that is mounted on a rotating gantry employs an inverter using high voltage asymmetrical silicon controlled rectifiers, currently available from only a single source.
The stored energy of a prior art power supply for gantry mounted X-ray tubes is on the order of 30 joules, resulting in large electric field stresses on the components in the power supply. The geometry to achieve this prior art power supply is difficult to reproduce, and results in high voltage gradients. This prior art power supply employs solid, potted dielectrics which are difficult to control in manufacture, and are susceptible to voids, corona failure, and cannot be repaired. Other parts of the prior art devices are also difficult to manufacture and repair. In addition, the time required for the prior art device to achieve full voltage is relatively great, being approximately 5 milliseconds.
The prior art device has poor tolerance to arcs which occur as a result of X-ray tube discharges. Typically, the voltage is not divided equally during an arc, with the highest voltage stages having a considerably larger percentage of the arc voltage developed across them than the lower voltage stages. Poor arc tolerance also occurs as a result of strong capacitive coupling between the primary and secondary windings. It is not possible to develop 150kV with a single prior art power supply because of limitations of potting and the requirements for a great number of multiplier stages. Control logic of the prior art device is relatively complex, requiring digital control of a voltage controlled oscillator loop for bridge selection and frequency adjustment to control the supply DC output.
The prior art power supplies have large shunt filter capacitors; some also have cables with significant shunt capacitance. The resulting large shunt capacitance results in an appreciable time for the voltage of the supply to be reduced from the rated value to zero volts. This has an adverse effect on human and X-ray tube safety.
Temperature variations of the prior art supply can result in the dielectric potting becoming cracked and can create gaps or voids at component surfaces, resulting in corona discharge and failure of the supply and possibly of the tube connected to it.
While the size and weight problems can be reduced by providing a power supply having a transformer driven by a high frequency source, the typical prior art approaches to high frequency have not been effective for the high voltage, high power application because of the high parasitic capacitance reflected from the secondary winding to the primary winding. In a high voltage power supply, there is an additional constraint imposed by dielectric strength and derating of materials, i.e., many materials that are rated for a certain voltage cannot be used for that voltage (many materials rated for 100kV are not used in greater than 10kV environments.) In high voltage transformers, size is frequently determined by puncture and creep considerations, rather than by thermal or flux density limitations.
My analysis indicates that the problem in size and weight reduction in high voltage, high power power supplies must be solved by using a system having low dielectric stress, increased operating frequency (in excess of about 100kHz) and a high bandwidth step-up transformation device permitting the use of high frequencies. It is, for example, desirable for the converter, including a power control feedback loop, to have a bandwidth approaching 20kHz. In addition, it is desirable, in certain situations, for a high voltage, high power power supply to be driven by portable power generating devices, such as a pair of automotive batteries connected in series. It is also desirable for the ripple of the high voltage derived by the supply to be relatively low. Low frequency ripple is essential. For the X-ray application, high frequency ripple, in excess of about 200kHz, can be tolerated to a certain extent.
Apparently, one reason for the slow acceptance of grounded anode X-ray tubes has been the unavailability of a -150kV power supply, in combination with the requirement for a filament power supply for the X-ray cathode that floats at the -150kV level.
It is, accordingly, another object of the present invention to provide a new and improved X-ray assembly that is capable of supplying a cathode of an X-ray tube used for CT applications with a voltage of -150kV, while simultaneously supplying current to a filament for the cathode at a voltage that floats at approximately the same voltage as the cathode.
Another object of the invention is to provide a new and improved X-ray tube assembly wherein a rotating gantry carries an X-ray tube for CT applications, as well as the high power, high voltage power supply for the X-ray tube and a slip ring assembly for supplying relatively low voltage excitation to the power supply.
It is still a further object of the invention to provide a new and improved module or assembly for a high voltage, high power DC power supply, which module develops a portion of the DC voltage derived by the supply, while assisting in minimizing the cost, weight, as well as size of the supply, and contributing to safety enhancement as well as ease of manufacture and repair.